Method and system for providing current limiting controllers for high reactance permanent magnet generators

ABSTRACT

A system and method is provided for overload and fault current protection of a permanent magnet generator. The system comprises a permanent magnet machine, a DC link, an inverter, coupled between the permanent magnet machine and the DC link, and a controller adapted to provide a linear decrease in voltage for the DC link when the permanent magnet machine is in at least one of an overload and fault current condition.

CROSS REFERENCES TO RELATED APPLICATIONS

[0001] Related subject matter is disclosed in a U.S. patent applicationof Kalman et al. entitled, “Permanent Magnet Generator and GeneratorControl”, Ser. No. 09/746,437, filed on Dec. 21, 2000, the entirecontents of which being incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates generally to electrical machines.Specifically, the invention relates to a method and system for providingoverload protection and fault current coordination for a high reactancepermanent magnet generator.

BACKGROUND OF THE INVENTION

[0003] Many airplanes have high-speed electrical generators that areused for generating power during flight. The electrical generatorsgenerate AC power, which is converted to DC power. The DC power is thensupplied through a DC distribution system to on-board electronics suchas radar, vapor cycle compressors, flight control electronics,electromechanical/electro-hydrostatic actuators, and the like. Theelectrical generators can be wound field synchronous machines, switchedreluctance machines, permanent magnet machines, or other types ofmachines.

[0004] Conventional wound field generators can provide short circuitprotection. However, conventional wound field generators areincompatible with high-speed prime movers unless a gear reduction stageis added. Unfortunately, the gear reduction stage adds cost andcomplexity to the airplane design.

[0005] For high-speed applications, the permanent magnet machine isdesirable because of its robust rotor design and a low magnetic springrate associated with its large air gap. However, the permanent magnetgenerator's excitation is fixed which does not provide for a “gracefulsurvival” of short circuit conditions.

[0006] For example, because of their low impedance, short circuitcurrents in excess of the permanent magnet generator's current ratingcan flow, causing excessive heat build up in the generator's statorwindings. A short at the terminals of the machine, or at the DC link canliterally melt the windings and destroy the generator. Thus, if agenerator cannot survive the short circuit conditions, the generatorcannot recover and deliver power to the aircraft when the short circuitis removed.

[0007] There is a need for a high-speed permanent magnet generator thatcan gracefully survive short circuit conditions.

SUMMARY OF THE INVENTION

[0008] An object of the present invention is to provide a high reactancepermanent magnet generator (HRPMG) that when combined with appropriatepower electronics and control system enables the HRPMG to provide powerto the electrical distribution system during both normal and abnormaloperation.

[0009] Another object of the current invention is to minimize the totalKVA rating of all the power handling components of the system, and toenable the power generation system to sustain short circuit conditionswithin these KVA limitations.

[0010] Still another object of the present invention is to provide alinear decrease in voltage for the HRPMG during an overload period.

[0011] These and other objects are substantially achieved by providing asystem and method for providing overload and fault current protection toa permanent magnet generator. The system comprises a permanent magnetmachine, a DC link, an inverter, coupled between the permanent magnetmachine and the DC link, and a controller adapted to provide a lineardecrease in voltage for the DC link when the permanent magnet machine isin at least one of an overload and fault current condition.

BRIEF DESCRIPTION OF DRAWINGS

[0012] The details of the present invention can be readily understood byconsidering the following detailed description in conjunction with theaccompanying drawings, in which:

[0013]FIG. 1 is a block diagram illustrating an example of a currentlimiting controller for a permanent magnet generator in accordance withan embodiment of the present invention;

[0014]FIG. 2 shows a graph for providing overload and fault current(OFC) protection in accordance with an embodiment of the presentinvention;

[0015]FIG. 3 is a simplified block diagram illustrating an example of acurrent limiting controller for a permanent magnet generator inaccordance with an embodiment of the present invention;

[0016]FIGS. 4A through 4E together show phasor relationships amongfirst, second and third operating points in accordance with anembodiment of the present invention;

[0017]FIGS. 5A through 5H are graphs showing transient responses of thecurrent limiting controller to increasing loads in accordance with anembodiment of the present invention; and

[0018]FIG. 6 is a graph showing an over-modulation condition for thecurrent limiting controller in accordance with an embodiment of thepresent invention.

[0019] To facilitate understanding, identical reference numerals havebeen used, where possible, to designate identical elements that arecommon to the figures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020]FIG. 1 is a block diagram illustrating an example of a currentlimiting system 10 for a permanent magnet generator 12 in accordancewith an embodiment of the present invention. The system 10 comprises thepermanent magnet electrical generator 12, which is preferably a highreactance permanent magnet generator (HRPMG) with a rotor 14, threephase stator windings 16, with associated inductance and resistancerepresented as an inductance 18, and a resistance 20. PMG terminals 21,AC current sensors 22, a power inverter 24, a DC link 26, a DC linkvoltage sensor 28, a DC link capacitor 29, a DC load current sensor 30,and a rotor position sensor 32. The system 10 also comprises a currentlimiting controller 33 which further includes an inner current loopoperating in the synchronous reference frame of the machine rotor 34, afirst complex block 36, an inner current loop block 38, a first summingjunction 40, a first proportional integral (PI) regulator 42, a secondcomplex box 44, a space vector modulation (SVM) logic block 46, a lowwins block 48, an outer loop block 50 including a second PI regulator52, a third PI regulator 54, a second summing junction 56, a thirdsumming junction 58, a first non-linear function generator 60, and asecond non-linear function generator 62.

[0021] The power inverter 24 is disposed between the PMG terminals 21and the DC link 26. The power inverter 24 is preferably a three-phasebridge inverter and includes six switches: a set of upper switches (notshown) and a set of lower switches (not shown). Each switch of the powerinverter 24 includes a controllable solid-state device (not shown)(e.g., an IGBT, a MOSFET, and the like) and an anti parallel diode (notshown) across the solid-state device. The DC link capacitor 29 iscoupled across the DC link 26 to provide a low source impedance for thepower inverter 24, and to reduce voltage ripples on the DC link 26.

[0022] It should be appreciated by those skilled in the art thatalthough the system 10 is described in connection with three-phase ACpower, it is not so limited. For instance, the system 10 can utilizetwo-phase AC power.

[0023] The DC link 26 is connected to the DC link voltage sensor 28 andto the DC load current sensor 30. The power inverter 24 provides anoutput voltage, vdc, to the DC link voltage sensor 28 and a loadcurrent, iLD, to the DC load current sensor 30. When a load is appliedto the distribution system, the reduction in the output voltage of thepower inverter 28, causes the control system to extract energy from theHRPMG in order to return the DC link voltage to its regulated value.When a short circuit is applied to the DC link, the current in themachine is limited only by the machine internal impedance, shown as alumped parameter inductance 18, and resistance 20.

[0024] The three phase stator windings 16 include inductance 18 andresistance 20. The three phase currents are sensed by the machinecurrent sensors 22, and the individual currents are combined to form aPark Vector or space vector in block 36, which is then transferred tothe synchronous reference frame based on the position sensor 32. Therotor position sensor 32 is coupled to the rotor 14 and provides rotorposition information to the vector co-ordinate transformation block 34.Specifically, the co-ordinate transformation block 34 transforms thecurrent vector in stationary co-ordinates, represented by sinusoidallyvarying time displaced currents, into a current vector in thesynchronous reference frame where the currents are basically DC values.

[0025] The voltage generated from the three phase stator windings 16 ofthe generator 12 also known as the “back emf” will be referred to as thegenerator voltage or generator emf. The voltage at the terminals of thepower inverter 24 will be referred to as the inverter terminal voltage.Reference is now made to FIG. 2 which shows a graph for providingoverload and fault current (OFC) protection in accordance with anembodiment of the present invention. The y-axis and x-axis of the graph64 describe the DC link voltage in volts and the DC load current inamps, respectively. When the power generation system is regulating theoutput voltage at its rated value, 270 VDC, as the load is increasedupon the distribution system, the voltage is maintained at its regulatedvalue. Once the output power reaches the rated KVA of the system, theoutput voltage is reduced, but the current is maintained at anessentially constant value. In this way, the power generation system cansupply overload and short circuit current to the distribution systemwithout exceeding its KVA rating.

[0026] Referring to FIG. 1, the DC link voltage sensor 28 and DC loadcurrent sensor 30 provide a DC link voltage feedback signal and a DCload current feedback signal, respectively, to the outer loop block 50.The DC link voltage and DC load current feedback signals contain themeasured voltage for the DC link 26 and the measured current for theload. When a load is coupled to the DC link 26, current is provided tothe load. The system 10 insures that the appropriate voltage is providedto the load. If there is a short circuit on the DC link 26 or anoverload condition, the PMG 12 needs to be protected. Thus, the system10 provides overload and short circuit protection for the PMG 12.

[0027] Specifically, there are two paths from the outer loop 50 to theinner current loop in synchronous co-ordinate system. In path one, theDC load current sensor 30 provides the DC load current feedback messageto the first nonlinear function generator 60 which converts the DC loadcurrent feedback signal to a signal AMP representing an equivalent ACcurrent peak value, which is used as a machine current amplitudereference. The calculation of AMP is disclosed in US Patent ApplicationPublication No. US 2002/0110007 Al.

[0028] Path two is further divided into two sub-paths. In the firstsub-path, the DC link voltage sensor 28 provides the DC link voltagefeedback signal to the second nonlinear function generator 62 whichconverts the DC link voltage feedback signal to a DC load currentcommand signal. The DC load current command signal specifies the desiredload current for the DC link 26. The third summing junction 58determines a first error signal which is the difference between thedesired DC load current and the measured DC load current. The firsterror signal is provided to the third PI regulator 54. The third PIregulator 54 provides an ANG signal to the low-win block 48. The ANGsignal is the angle component for the first error signal.

[0029] In the second sub-path, the second summing junction 56 determinesa second error signal, which is the difference between the DC linkvoltage feedback signal and a 270 volt DC reference signal. The seconderror signal is provided to the second PI regulator 52 which processesthe received signal to provide an ANG signal to the low-win block 48.The low-win block 48 monitors the signals received from the second andthird PI regulators 52 and 54. When the signal from the third PIregulator 54 is lower than the signal from the second PI regulator, thelow win circuit 48 will output a command equal to the output of the PIregulator 54. When the output of PI regulator 52 is lower than theoutput of PI regulator 54, the output of the low wins circuit 48 is theoutput of PI regulator 52.

[0030] The rotor position sensor 32 generates a rotor position signalRTR which indicates the angle of the rotor 14 relative to the center ofthe “A” phase winding of the stator. The rotor position sensor 32 isaligned such that the unit vector e^((jRTR)) is in phase with the PMG 12voltage vector V_(EMF). The rotor position signal RTR provides theposition reference for the PMG's 12 EMF. AC current sensors 22 generatesignals I_(SEN) indicating current sensed at the PMG terminals 21.

[0031] The first complex block 36 combines the sensed current signalsI_(SEN) to form a current Park vector in the stationary reference frame,which is then transformed to the synchronous reference frame withrespect to the rotor 14 using the unit vector e^((−jRTR)). The currentfeedback vector I_(FBK) so formed is supplied to the inner loop 38.

[0032] Park vectors inherently contain information on both theinstantaneous magnitudes and the phase relationship of three phaserotating fields with respect to a reference coordinate system. A Parkvector, in general, is a mathematical representation that describes thelocus of an electrical quantity in the complex space domain where timeis a parameter. A voltage Park vector is defined with the vector'samplitude and the vector's direction in spatial relation to the threephases. A general discussion of Park vectors is provided in P. K.Kovacs, “Transient Phenomena in Electrical Machines,” Elsevier SciencePublishing Co. (1984), which is incorporated herein by reference.

[0033] The current Park vector may be converted from a stationary frameto a synchronous frame by extracting a unit amplitude Park vector,e^(jRTR), from the rotor position signal RTR (e.g., by computing thefunction cos(RTR)+jsin(RTR)) and taking the product of the current Parkvector and a complex conjugate, e^(−jRTR), of the unit amplitude Parkvector. The synchronous current Park vector I_(FBK) is synchronous withrespect to the rotor 14. The unit amplitude Park vector is representedby a complex number in polar coordinates or as a+ib in Cartesiancoordinates from which the complex conjugate, e^(−jRTR), of the unitamplitude Park vector is determined.

[0034] The second complex block 44 combines the amplitude magnitudesignal AMP and the load angle signal ANG to produce a vector-basedcurrent command, e.g., AMPe^(−jANG), from the signals ANG and AMPindicating load angle and amplitude. Thus, the load angle signal ANGprovides the angle-portion of the vector-based current-command, and thecurrent peak value represents the amplitude portion of the vector-basedcurrent command. An output of the second block 48 supplies the currentcommand ICMD so formed to the inner loop 38.

[0035] The inner loop 38 includes a first summing junction 40, whichsubtracts the feedback current vectors I_(FBK) from the current commandI_(CMD) to produce an error vector I_(ERR). The vector-based PIregulator 42, which is preferably a current regulator, with appropriateamplitude limits, converts the error vector I_(ERR) into a voltagevector command V_(CMD) in the synchronous reference frame. This voltagevector is then transformed back to the stationary reference frame bymultiplication by e^(jRTR). The voltage vector command V_(CMD)represents the voltage amplitude and the load angle of the inverter's 24terminal voltage and provides appropriate AC voltage at the PMGterminals 21. The amplitude limits are established by the inverter dutycycle range for the desired DC link voltage.

[0036] By controlling both the current amplitude, AMP, and theassociated command angle, ANG, the PMG's 12 current can be minimized forany applied load on the DC link 26. In other words, the AMP and ANGsignals are adjusted with respect to the EMF of the PMG 12.

[0037] The SVM logic 46 converts the voltage vector command VCMD backinto a stationary reference frame, e.g., by taking a product of thevoltage command VCMD and the unit amplitude Park vector e^(jRTR), anduses space vector modulation to turn the switches of the inverter 24 onand off. The switches of the inverter 24 are modulated at a highfrequency, e.g., 40 kHz in order to minimize the size of the DC linkcapacitor 29.

[0038] It should be appreciated by those skilled in the art that the PIregulators 40, 52 and 54 are application specific. The regulator valuesare preferably dependent upon desired responses to the power inverter24, DC link voltages and other system constraints and inputs.

[0039]FIG. 3 is a simplified block diagram illustrating an example of acurrent limiting controller for the permanent magnet generator 12 inaccordance with an embodiment of the present invention. Specifically,FIG. 3 is a simplified block diagram of FIG. 1. The control system 64provides voltage control and overload protection for the PMG 12. Thecontrol system 64 interacts with the power inverter 24 to control the DClink voltage.

[0040]FIGS. 4A through 4E together show phasor relationships amongfirst, second and third operating points in accordance with anembodiment of the present invention. FIG. 4A depicts a portion of thesystem 10 including the PMG 12, windings 16, inverter 24, DC link 26,and capacitor 29 with reference to the directions of currents andvoltages on the DC and the AC side of the inverter 24. The AC side ofthe circuit is located between the PMG 12 and the inverter 24. The DCside of the circuit is located on the DC link 26 side of the inverter ofFIG. 4A. The PMG's 12 current, terminal voltage and EMF represented byphasors i, u, and e, respectively are disposed on the AC side of thecircuit. The DC current, DC voltage and DC current load represented byid, vdc and iLD, respectively are disposed on the DC side of the circuitof FIG. 4A.

[0041]FIG. 4B shows the overload and fault current line 68 of FIG. 2having operating points 1, 2 and 3. Operating points 1, 2 and 3represent the 100%, 50%, and 0% DC link 26 voltage conditions. In otherwords, in the overload and fault current region of FIG. 4B, the loadvoltage is reduced so that it preferably follows OFC line 68. Ratherthan going from 100% to 0% almost instantaneously, the voltage isreduced gradually, providing a graceful degradation.

[0042]FIG. 4C shows a phasor diagram when the system 10 is at operatingpoint 1. The reactive drop of the PMG 12, i.xG, represented by plot 70is constant. The locus also known as the “path followed by” of the EMFis also constant and is represented by arc 72. The locus of the current,i represented by arc 74 is also constant. The locus of the PMG's 12current, i, is a circle since along the OFC line 68 the system 10preferably requires a constant current amplitude equal to the PMG's 12short circuit current. The angle δ changes for the different operatingpoints since the reactive drop remains constant.

[0043]FIG. 4D shows a phasor diagram when the system 10 is at operatingpoint 2. The DC link voltage is reduced 50%. In response, the PMG's 12terminal voltage, u, also decreases while the angle δ increases.

[0044]FIG. 4E shows a phasor diagram when the system 10 is at operatingpoint 3. The DC link voltage is at or near zero and the PMG's 12terminal voltage, u, has decreased further from that shown in FIG. 4D.In comparing FIGS. 4D through 4E, it can be seen that with respect tothe EMF vector e, at 100% DC link voltage the current vector hassubstantially a direct axis content. However, at or near 0% DC linkvoltage, the current vector primarily has quadrature axis currents asshown in FIG. 4E.

[0045]FIGS. 5A through 5H are graphs showing transient responses of thecurrent limiting controller to increasing loads in accordance with anembodiment of the present invention. The graphs show that therequirements of FIG. 2 with reference to the OFC line 68 have been met.

[0046] In FIG. 5A, plot 76 shows that the DC link voltage decreaseslinearly when the system 10 is in an overload and fault currentcondition. Similarly, plot 78 shows that the current increases linearlywhen the system 10 is in an overload and fault current condition.

[0047] The plot 80 in FIG. 5B illustrates how the system responds to anincreasing load current by substantially matching the OFC line 68. InFIG. 5C plot 82, illustrates that the locus of the generator currentremains a circle while the currents direct and quadrature componentschange. In FIG. 5D plots 84 and 86, illustrate that to maintain aconstant current amplitude the load angle preferably increases as the DClink voltage decreases.

[0048] In FIG. 5E plots 88 and 90 and in FIG. 5F plots 92 and 94illustrate how the direct and quadrature axis currents and SVM voltagecommand components vary.

[0049] In FIG. 5G plots 96 and 98 illustrate how the system responds tothe decreasing DC link voltage by increasing the DC link current.

[0050] In FIG. 5H plot 100 illustrates that the modulation of the SVM 46limits the DC link current that the system 10 provides when themodulation index is saturated.

[0051]FIG. 6 is a graph showing an over-modulation condition for thecurrent limiting controller in accordance with an embodiment of thepresent invention. Specifically, FIG. 6 shows over-modulation for theSVM logic block 46. In order to control the current voltage curve fromthe full power to the short circuit condition, the SVM logic block 46preferably goes into an over-modulation mode. The over-modulation modeis preferably nonlinear.

[0052]FIG. 6 also illustrates a single 60-degree portion of the SVMlogic block's 46 complex plane. The vectors v1 and v2 represent two outof six possible, nonzero inverter voltage park vectors. The commandvoltage vector is preferably a linear combination of the v1, v2 and v0vectors. To avoid saturation of the v_(CMD) signal, the locus of thev_(CMD) amplitude is limited to preferably a 0.577×VDC value.Over-modulation suppresses v0. Thus an extra 15% SVM logic block 46voltage can be obtained by increasing the maximum VCMD value from0.577×VDC to 0.666×VDC.

[0053] Typically, control systems operate within area 102 which is thelocus of maximum vCMD without over-modulation. However, control system10 operates within area 104 which is the locus of maximum vCMD withover-modulation in order to control line 68 in FIG. 2A. This allows thecontrol system 10 to utilize the full capability of the inverter 24.

[0054] Thus, disclosed is a permanent magnet generator 12 that cangracefully survive short circuit conditions. Such a permanent magnetgenerator 12 and control system 10 can be used to generate DC power inaircraft. Further, short circuit currents can be supplied by such asystem without exceeding the rated current for the permanent magnetgenerator 12 and the inverter 24.

[0055] Those skilled in the art can now appreciate from the foregoingdescription that the broad teachings of the present invention can beimplemented in a variety of forms. Therefore, while this invention canbe described in connection with particular examples thereof, the truescope of the invention should not be so limited since othermodifications will-become apparent to the skilled practitioner upon astudy of the drawings, specification and following claims.

What is claimed is:
 1. A system comprising: a permanent magnet machine;a DC link; an inverter, coupled between said permanent machine and saidDC link; and a controller adapted to provide a linear decrease involtage for said DC link when said permanent magnet machine is in atleast one of an overload and fault current condition.
 2. The system ofclaim 1, wherein said controller regulates a DC link voltage byadjusting a load angle and magnitude of an AC terminal voltage of saidinverter with respect to an electromagnetic force (EMF) of saidpermanent magnet machine.
 3. The system of claim 1, wherein saidpermanent magnet machine comprises a high reactance permanent magnetmachine.
 4. The system of claim 1, wherein said permanent magnet machineis thermally rated up to 270 volts DC and up to 445 amps DC.
 5. Thesystem of claim 1, wherein said overload and fault current conditionsoccur when a load current is between 445 and 605 amps DC.
 6. The systemof claim 1, wherein said controller controls the DC link voltage as afunction of current when at least one of said overload and fault currentconditions exist.
 7. The system of claim 2, wherein the load angle istaken as the angle between a Park vector representing machine emf and aPark vector representing the ac terminal voltage.
 8. The system of claim2, wherein the controller provides a vector based current command signalas a function of the load angle and a permanent magnet machine referencesignal.
 9. The system of claim 8, wherein the controller provides afeedback current vector signal representing a detected current flowingthrough stator windings of the permanent magnet machine and the currentvector signal and the current command signal being synchronous withrespect to a rotor angle of the permanent magnet machine.
 10. The systemof claim 9, wherein the controller provides a voltage command signal tomodulate the inverter, the voltage command signal being derived from thevector-based current command signal and the feedback current vectorsignal.
 11. The system of claim 10, wherein said controller modulatesthe inverter using space vector modulation.
 12. A method of providingoverload and fault current protection to a permanent magnet machinegenerator, the method comprising: determining a load angle from ameasured DC link voltage and a reference DC link voltage; providing areference current signal from said load angle; detecting a measuredcurrent from an AC generator; providing a reference voltage signal fromsaid reference current signal; and modulating an inverter with saidreference voltage signal to provide a linear decrease in current of saidDC link when said permanent magnet machine is in one of at least anoverload and fault current condition.
 13. The method of claim 12,wherein the permanent magnet machine comprises a high reactancepermanent magnet machine.
 14. The method of claim 12, wherein thereference current signal is synchronous with respect to anelectromagnetic force (EMF) of the permanent magnet machine.
 15. Themethod of claim 12, wherein the measured current of the AC generator issynchronous with respect to an electromagnetic force (EMF) of thepermanent magnet machine.
 16. The method of claim 12, wherein saidpermanent magnet machine is thermally rated up to 270 volts DC and up to445 amps DC.
 17. The method of claim 12, wherein an overload and faultcurrent condition occur when a load current is between 445 and 605 ampsDC.
 18. The method of claim 12, wherein the inverter is modulated usingspace vector modulation.